Switchgear enclosures are commonly employed in electrical power distribution systems for enclosing circuit breakers and switching equipment associated with the distribution system. Typically, switchgear enclosures are comprised of a number of individual stacked or adjacent compartments, each of the switchgear compartments receiving electrical power from a power source and distributing the electrical power through a feeder circuit to one or more loads. Generally, each of the switchgear compartments includes circuit breakers for interrupting electric power in a particular feeder circuit in response to hazardous current overloads in the circuit.
Switchgear is a general term covering switching and interrupting devices and their combination with associated control, instruments, metering, protective and regulating devices, also assemblies of these devices with associated interconnections, accessories, and supporting structures used primarily in connection with the generation, transmission, distribution, and conversion of electric power. The following paragraphs describe switchgear characteristics in accordance with ANSI/IEEE Standards No. C37.20.2-1987.
A switchgear assembly generally refers to assembled equipment (indoor or outdoor) including, but not limited to, one or more of the following: switching, interrupting, control, instrumentation, metering, protective and regulating devices, together with their supporting structures, enclosures, conductors, electrical interconnections, and accessories. A switchgear assembly may be completely enclosed on all sides and top with sheet metal (except for ventilating openings and inspection windows) containing primary power circuit switching or interrupting devices, or both, with buses and connections referred to as metal-enclosed (ME) power switchgear. The assembly may include control and auxiliary devices. Access to the interior of the enclosure is usually provided by doors or removable covers, or both.
Metal-enclosed power switchgear may include one or more of the following features:
(1) The main switching and interrupting device is of the removable (drawout) type arranged with a mechanism for moving it physically between connected and disconnected positions and equipped with self-aligning and self-coupling primary disconnecting devices and disconnectable control wiring connections. PA1 (2) Major parts of the primary circuit, that is, the circuit switching or interrupting devices, buses, voltage transformers, and control power transformers, are completely enclosed by grounded metal barriers, that have no intentional openings between compartments. Specifically included is a metal barrier in front of or a part of the circuit interrupting device to ensure that, when in the connected position, no primary circuit components are exposed by the opening of a door. PA1 (3) All live parts are enclosed within grounded metal compartments. PA1 (4) Automatic shutters that cover primary circuit elements when the removable element is in the disconnected, test, or removed position. PA1 (5) Primary bus conductors and connections are covered with insulating material throughout. PA1 (6) Mechanical interlocks are provided for proper operating sequence under normal operating conditions. PA1 (7) Instruments, meters, relays, secondary control devices and their wiring are isolated by grounded metal barriers from all primary circuit elements with the exception of short lengths of wire such as at instrument transformers terminals. PA1 (8) The door through which the circuit interrupting device is inserted into the housing may serve as an instrument or relay panel and may also provide access to a secondary or control compartment within the housing. PA1 (1) Rated maximum voltage PA1 (2) Rated frequency PA1 (3) Rated insulation levels PA1 (4) Rated continuous current PA1 (5) Rated short-time current PA1 (6) Rated momentary current PA1 (1) Low frequency 1 min withstand voltage PA1 (2) Impulse withstand voltage PA1 (1) Any primary or secondary circuit component PA1 (2) Any insulating medium, or structural or enclosing member
Switchgear leaving all of the above eight features is referred to as metal-clad (MC) Switchgear. Metal-clad switchgear is metal-enclosed, but not all metal-enclosed switchgear is metal-clad.
The ratings of a switchgear assembly are designations of operating limits under specified conditions of ambient temperature, temperature rise, etc. Where the switchgear assembly comprises a combination of primary and secondary circuits, each may be given ratings.
ME switchgear usually has the following ratings:
The designated ratings in this standard are preferred but are not considered to be restrictive.
In addition to these ratings, a switchgear assembly may have interrupting or switching capabilities, which are determined by the rating of the particular interrupting and switching devices that are integral parts of the switchgear assembly.
The rated maximum voltage of ME switchgear is the highest arms voltage for which the equipment is designed, and is the upper limit for operation.
The rated insulation levels of ME switchgear includes two items.
The rated maximum voltages, and corresponding insulation levels for ME switchgear are listed in tabular form in ANSI/IEEE C37.30.2-1987.
The rated frequency of a device, or an assembly, is the frequency of the circuit for which it is designed. (Ratings are usually based on a frequency of 60 Hz).
The rated continuous current of ME switchgear is the maximum current in arms amperes at rated frequency, which can be carried continuously by the primary circuit components, including buses and connections, without causing temperatures in excess of specified limits for
The specified temperature limits applicable to switchgear assemblies are given in ANSI/IEEE C37.20.2-1987, .sctn..sctn.4.5.1 through 4.5.6.
The continuous current ratings of the main bus in ME switchgear are listed in ANSI/IEEG C37.20.2-1987.
The continuous current rating of the individual circuit-breaker compartment shall be equal to the ratings of the switching and interrupting devices used, except as may be modified by lower continuous current ratings for current transformers, power fuses etc.
The rated momentary current of ME switchgear is the maximum arms total current that it shall be required to withstand. The current shall be the arms value, including the direct-current component, at the major peak of the maximum cycle as determined from the envelope of the current wave during a test period of at lease 10 cycles unless limited to a shorter time by the protective device.
The momentary current ratings of the individual circuit-breaker compartments of ME switchgear shall be equal to: The circuit breaker close and latch, switch fault close, or asymmetrical momentary current ratings of the switching devices used.
The rated short-time current of the ME switchgear is the average arms current that it can carry for a period of 2 sec. unless limited to a shorter time by the protective device or current transformer ratings.
The short-time current ratings of the individual circuit-breaker compartments of the ME switchgear shall be equal to the short-time ratings of the switching and protective devices used or the short time rating of the current transformers (see ANSI/IEEE C57. 13-1978 (R 1986) [10]).
The limiting temperature for ME switchgear is the maximum temperature permitted.
In addition to current overloads, the switchgear enclosure may encounter other hazardous conditions known as arcing faults. Arcing faults occur when electric current "arcs" or flows through ionized gas between conductors, between two ends of broken or damaged conductors, or between a conductor and ground in the switchgear enclosure. Arcing faults typically result from corroded, worn or aged wiring or insulation, loose connections and electrical stress caused by repeated overloading, lightning strikes, etc. Particularly in medium- to high-voltage power distribution systems, the ionized gas associated with arcing faults may be released at pressures and temperatures sufficient to damage the switchgear equipment.
Presently, the most commonly employed method for enhancing the durability of switchgear enclosures in the event of arcing faults is to provide arc-resistant metal switchgear compartments to the above-described MC (metal clad) standards, with a means for venting the gases from the compartments in the event of an arcing fault. These compartments are designed to withstand the pressures and temperatures of the gases associated with an arcing fault and reduce the likelihood or extent of damage to switchgear equipment by preventing the gases from entering adjacent switchgear compartments. However, because these systems do not eliminate the generation and release of hot gases associated with arcing faults, they do not eliminate the risk of damage to the switchgear equipment.
Cronin, U.S. Pat. No. 4,130,850 is directed to a high speed fault diverter switch for a gas-insulated substation. However, the switch referred by Cronin is a High Voltage switch which would be used in a GIS (Gas Insulated Substation). The lowest typical voltage application for this type of system would be at 60-145 kV which requires an impulse voltage level of up to 650 kV. For this application and in order to withstand the high voltage, the open gap of the contacts would be typically around 4 inches. However, Cronin discloses only a conventional switch, which would require 3 to 4 cycles (i.e., 48-64 in sec.) to operate. Cronin speaks of the rise of the high pressure being extremely rapid; indeed, experience has shown that the arc should be controlled within 4 milliseconds.
Therefore, the conventional switch will not work, because the contacts must travel 4 inches in 4 milliseconds which gives an approximate average velocity of 25 meters per second. Since, the acceleration time is 4 ms. and the initial velocity is 0 then the required constant acceleration is about 12,500 meter second squared.
And when the mass of the contact and the drive system is taken into consideration (assume a typical mass of 6 kg) the required force is 75 kilo-newtons which for the required 4 inch or about 100 mm stroke gives an energy requirement of 7.5 kilo-newton-meter. Presently available switch mechanisms are incapable of fulfilling this requirement, in fact, they are at least one order of magnitude short of this level of energy. Therefore, Cronin presents a problem without a practical solution. That is, the switch required by Cronin does not exist.
The use of the electrodynamic drive would not be feasible either because the discharge pulse is generally in the order of 1 to 2 milliseconds. For example, the discharge pulse described in Diebold U.S. Pat. No. 2,971,130, is a 1 millisecond pulse. With a 1 ms pulse discharge and assuming that the transfer efficiency of the system is 20% at best, the components required can be estimated. Energy from the capacitor is E2C=30,000 newton-meters. A reasonable voltage to charge the capacitor would be 3,000 volts in which case a capacitor of 0.0033 farads is needed (this is a rather large capacitor) or if a more common capacitor of 100 micro-farads is used then the charging voltage would be 300 kV.
Accordingly, there is a need for a system of protecting switchgear enclosures and from arcing faults in a manner which is very rapid and reduces or eliminates the generation of ionized gases at high temperatures and pressures. The present invention is directed to addressing this need.